The long term goal of the proposed research is to understand at a molecular level how temporal developmental pathways are specified, how a choice is made between pathways, and the mechanisms for regulation of gene expression and site-specific recombination. The temperate bacterial virus Lambda has been chosen as the ideal agent for a study of these questions in a relatively simple system amenable to combined biochemical and genetic approaches. Specific objectives of the new grant are to learn: (1) the regulatory events that provide for the switch between the lytic and lysogenic pathways of viral development; (2) the mechanisms responsible for site-specific recombination and its directional control. Work on the switch mechanism for lysis or lysogeny will focus primarily on the molecular basis of the recently discovered post-transcriptional controls that involve the synthesis and stability of the Lambda cII, cIII, and Int proteins. The experiments will use purified viral and host proteins, guided by old and new genetic analysis. This aspect of the work will also include a collaborative X-ray crystallography project with Brian Matthews at Oregon on the structure of the cII transcription regulator and its interaction with DNA. The study of site-specific recombination will seek to define the molecular pathways of integrative and excisive recombination and how the direction of the reaction is controlled. Purified viral and host proteins will be used, and their interaction with DNA substrates probed in detail. Another collaborative study with Matthews will try to determine the structure of Int and its multiple interactions with DNA.